Would this work!?
Consider this:
The pentagram temple is about 520 sq ft about 40 ft high. Thats about 20,800 cubic ft of volume.
A Decanter of Endless water can produce 30 gallons of water per round, which is a little less than 4 cubic feet.
For the low low price of around 99,000 GP, you can craft 22 of these babies, producing 660 gallons of water or about 88 cubic feet per round. In 10 mins, you can fill 880 cubic feet, and in about 23 minutes, you can fill the entire temple.
Now throwing the decanters in the temple and closing the door is likely not a good strategy. They could destroy the decanters in the pit of lava, or find another way of disposing them.
Instead, you could seal of a large area outside of the temple and fill it with water. Then open the portcullis to the temple, and the water will flow in filling much of the room, lets say 10 feet high. Seal off the corridor to the temple with Stone Shape or Wall of Stone and leave small 1ftx1ft holes in the wall. Re-activate the decanters and the water will flow through the holes, but no enemies can pass through the wall, and they will be slowed.
The Demon will have to try to break the wall(s) down at Hardness 8, 90 HP, Break DC 35 per wall you create. At this point the incorporeal spectre may come to fight, at least he'll be singled out. Cast silence behind the wall and don't worry about casters, they'll be swimming anyway.
In another 17ish mins the temple should be full of water. Someone can also cast Control Water, at lvl 8 you can raise the volume of water of an 80 square ft surface by 16 ft.
Submerged in water the casters will be unable to speak and therefore unable to cast. Movement will be at 1/4 speed, attacks will be at a -2, and will do half damage.
With a Constitution of say 15, they can hold breath for 10 rounds, then each check DC will increase by 1. About five rounds later when most enemies fail, in one round they will be unconcious, two rounds later they will drown.
At most, in 23 minutes you can fill the temple with water, within 3 minutes of being submerged, every enemy will be dead.
So cray it just might work??.
Might work, but it seems to me that a Hardness 8 wall is not going to resist high-level melee attacks for very long. Break through the wall, let the water drain out.
Also wonder what the enemies will be doing while the party is setting this up.
There's an AP that contains a ring gate, with the other end being underwater. Assuming a 6ft diameter aperture and that the other gate is located 2000 ft below sea level.... that should fill the entire temple in like 3-4 seconds? 890 psi is a LOT of pressure.